In an internal combustion engine such as an automobile engine, a Ni alloy or the like is generally used as a material forming a center electrode and a ground electrode. The Ni alloy is slightly inferior to a noble metal alloy containing a noble metal such as Pt or Ir as a principal component. However, the Ni alloy is suitably used as a material forming a ground electrode and a center electrode since Ni is cheaper than a noble metal.
Meanwhile, when spark discharge is caused between a front end portion of a ground electrode and a front end portion of a center electrode that are formed from a Ni alloy or the like, the respective opposed front end portions of the ground electrode and the center electrode are likely to cause spark wear. Thus, a method may be adopted in which a tip made of a noble metal is provided at each of the opposed front end portions of the ground electrode and the center electrode such that spark discharge is caused at the tip, thereby improving the wear resistance of the ground electrode and the center electrode.
Examples of a material forming the tip include Ir, an Ir alloy, and a Pt alloy (e.g., Japanese Patent Application Laid-Open (kokai) No. S58-198886). A method for joining the tip to each of the center electrode and the ground electrode is generally resistance welding. However, the joining strength may be insufficient.
For example, International Publication No. 2010/058835 discloses a spark plug for an internal combustion engine, “comprising: . . . a plate-like relieving layer tip joined to a front end portion of the ground electrode by means of resistance welding so as to be embedded therein; and a noble metal tip having one end surface that is joined by means of resistance welding to both a portion of the relieving layer tip at the center electrode side and a portion of the ground electrode at an outer peripheral side of a portion of the relieving layer tip at the center electrode side, the noble metal tip having another end surface forming a gap with a front end portion of the center electrode, wherein the noble metal tip is formed from a platinum alloy containing platinum as a principal component, the relieving layer tip is formed from a platinum alloy having a linear expansion coefficient between those of the platinum alloy forming the noble metal tip and a metal material forming the ground electrode, a portion of the relieving layer tip that is joined to the noble metal tip has an area smaller than an area of the one end surface of the noble metal tip, and a melt portion formed by melting at least the noble metal tip and the ground electrode by laser welding is provided on an entire outer periphery of a boundary portion between the ground electrode and the noble metal tip” (claim 1 of International Publication No. 2010/058835).
Meanwhile, in recent years, regarding an internal combustion engine such as an automobile engine, for achieving high output and improvement of fuel economy of the internal combustion engine, there is an increasing tendency to adopt an engine that directly injects fuel to the surrounding of a spark plug provided within a combustion chamber, or operate the internal combustion engine under a combustion condition in a high oxygen atmosphere such as lean burn. Under such a condition, a tip easily wears due to oxidation. Thus, a tip formed from an alloy containing Pt or Rh which has more excellent oxidation resistance than an Ir alloy or the like which has excellent spark wear resistance, is suitably used.
However, it has been found that, as shown in FIG. 7(a), such a tip formed from an alloy containing Pt and Rh bulges at a center portion thereof to deform into a convex shape under a specific condition in which a severe thermal cycle is repeated, whereby a spark discharge gap reduces to decrease ignitability or the tip peels off an electrode.
The above phenomenon more easily occurs as thermal stress generated in the tip and the electrode is greater. For example, in the case where the tip is joined to the electrode by means of laser welding, thermal stress can be relieved at a melt portion formed by the laser welding, so that the above phenomenon does not occur. Thus, it is conceivable to perform laser welding instead of or in addition to resistance welding, thereby relieving thermal stress to prevent occurrence of the above phenomenon. However, when the tip is joined to the electrode by means of laser welding, the performance as an electrode may diminish, conversely. For example, when a tip having a low height and a large width is joined to an electrode by means of laser welding, a melt portion is exposed in a discharge surface of the tip that contributes to spark discharge, so that the tip easily wears at the melt portion. As described above, under a specific condition such as the case of joining a tip having a low height and a large width to an electrode, the above problem cannot be solved by joining the tip by means of laser welding. Thus, a solution by other means is desired.
An advantage of the present invention is a spark plug that is able to suppress deformation and peeling of a tip, joined by a method different from laser welding, which occur under a specific condition in which a severe thermal cycle is repeated, and a method for producing the spark plug.